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Thermal imaging goes mechanical

The bimetallic strip is back, and it may be in your next thermal imaging …

Infrared detectors suck. The previous statement doesn't need any qualification at all. The most common method to detect light is to let the incoming photon excite an electron in some fashion. This creates a current that is proportional to the number of photons hitting the detector. This approach works extremely well for visible and near-infrared wavelengths. However, as the photon energy goes down (i.e., the wavelength gets longer), so too must the amount of energy required to excite the electrons. Such materials generate a large false signal because the thermal noise within the detector is sufficient to excite electrons. To make these detectors useful, researchers often cool them to liquid nitrogen temperatures or even liquid helium temperatures, which is a major inconvenience.

At even longer wavelengths, it is better to choose a material that absorbs everything and monitor the temperature of the material. These detectors are called bolometers and are commonly used in astronomy and thermal imaging gear. Unfortunately, the standard method for creating the imaging sensors leads to large pixels, a slow response time and a clunky, low resolution image. Clearly there is a need for new ways to construct thermal image sensors.

Now some new research has shown that bimetallic strips, the very same that your thermostat relies on, may be the way to go. The researchers constructed a sensor of 256 X 256 gold and chromium strips with an absorber attached to each strip. When thermal radiation falls on the absorber it heats up, causing the strip to bend. The amount of bending is measured by a read-out laser. The mass of the absorber is so small that the strip recovers in about 6ms, which is fast enough for the standard 30fps used in cameras. More importantly, there are no electronics attached to this baby at all, so the pixel density and array size can be increased without adversely affecting readout times. The researchers anticipate that they will be able to manufacture arrays in excess of 2000 X 2000 pixels, which is about a factor of four better than currently available.

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He is delocalised, living and working in Eindhoven and Enschede, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku